It began, improbably, in Wirral. Selwyn Lloyd, the foreign secretary, was visiting his Cheshire constituency on October 21 1956 when he was summoned to Chequers and ordered to take part in a secret meeting that will forever be associated with a fateful lie. Lloyd's mission lay at the heart of the Suez crisis, which rocked the Middle East, British politics and the world half a century ago.

The next day Donald Logan, Lloyd's private secretary, drove his boss incognito to Hendon to take an RAF flight to an airfield outside Paris. The cover story for Foreign Office officials was that Lloyd had a cold: their destination was a villa in the suburb of Sèvres, where, with representatives of France and Israel, they hatched a conspiracy to attack Egypt - and made the mistake of writing it all down.

Anthony Eden, the Tory prime minister, ordered the evidence destroyed. But there were leaks right from the start and when the proof emerged 40 years later - a smoking gun if ever there was one - Sèvres became, in the words of one historian, "the best-documented war plot in modern history".

Mr Logan, now Sir Donald, who at 88 is the last British survivor of the meeting, recalls that Lloyd was miserable. "He was desperately unhappy and uncomfortable at being given this job," he told the Guardian.

It meant the end of the diplomatic moves Lloyd had been orchestrating since July 26, when Gamal Abdel Nasser's nationalisation of the Suez Canal company triggered a crisis that threatened, as Eden saw it, to undermine Britain's position in the region, endanger Europe's oil supplies and advance Soviet interests. Eden was obsessed with the Egyptian leader, whom he often compared to Benito Mussolini.

"The seizure of the Suez canal is the opening gambit in a planned campaign to expel all western influence and interests from Arab countries," he wrote to Dwight Eisenhower, the US president, in September. The danger was "a united Arabia led by Egypt and under Russian influence". Failure to take action would be disastrous. But the US, the president said, wanted a peaceful solution.

It was not until mid-October that war became inevitable, thanks to an intimate alliance between the French and the Israelis, who were itching for a preventive war with Egypt after a string of border raids over the preceding months. Guy Mollet, the socialist prime minister, was desperate to stop Egypt backing the rebellion in Algeria.

Contact was maintained through a clandestine back-channel between Paris and Tel Aviv. France's defence minister, Maurice Bourgès-Maunoury, was a wartime resistance hero. Christian Pineau, the foreign minister, had been in Buchenwald. Eight years after abandoning Palestine, Britain, by contrast, was deeply suspicious of any link with Israel - and planned to attack it in case of war with Jordan. The French, in the words of historian Avi Shlaim, were the "matchmakers".

Eden had made up his mind on October 14 at a secret meeting with two French envoys whose names were excised from the visitors' book at Chequers. The prime minister told his private secretary, Guy Millard, not to take notes. "There'll be no need for that, Guy," he ordered.

Eight days later, when Lloyd arrived at a villa in rue Emmanuel Girot, a resistance safe house during the Nazi occupation, talks were already under way. Israel was represented by the prime minister, David Ben-Gurion, who was deeply suspicious of the British, and his defence minister, Moshe Dayan, who sketched military plans on the back of a cigarette packet. Another of the surviving participants, Dayan's aide Mordechai Bar-On, said Lloyd "looked as if he found the whole thing deeply distasteful".

On October 24, after Pineau had been to London to see Eden, the parties drew up a protocol - typed in French in the villa's kitchen - in which Israel undertook to attack Egypt, and Britain and France to invade on the pretext of "separating the combatants" and protecting the canal. That transparent fiction was at the heart of the famous "collusion" that will always be a byword for Suez. The Israelis were to get naval and air support while they dealt with the Egyptians.

Champagne

The document was supposed to remain "strictly secret". It was initialled for Britain by Patrick Dean, a senior FO official, who found the atmosphere "correct but not very friendly". Things lightened up over champagne and Franco-Israeli talks about nuclear cooperation, once he and Mr Logan had left. "We and the French were the buddies," said Mr Bar-On. "The British were the outsiders."

Back in London, Eden ordered the protocol destroyed. The cabinet secretary, Norman Brook, burned the British copy and other incriminating material in his Downing Street fireplace -and ordered the two diplomats back to Paris the next day to persuade the French do the same. But after being locked up for hours in a Quai d'Orsay reception room they failed to convince Pineau. "We got a flea in our ear," recalled Sir Donald. And the Israelis had already gone home with their copy.

Eden's secret was far from safe. From the moment war began with an Israeli attack in the Sinai desert on schedule on October 29, many assumed it had all been prearranged. "Stories of collusion between French and ourselves and Israelis are spreading widely, and ... a member of CIA has claimed they have evidence," the British embassy in Washington cabled Lloyd. "This reinforces need for urgent action to convince the administration that this is not true." But it was true, and a livid Eisenhower knew it. "Nothing justifies double-crossing us," he told his secretary of state, John Foster Dulles.

The government stuck to its guns. Lloyd was asked in parliament whether there had been collusion. "There was no prior agreement between us about it," he lied. On December 20 Eden was even more emphatic in the Commons. "There was no foreknowledge that Israel would attack Egypt - there was not." It was the last time he spoke in the house.

Mr Bar-On kept a detailed record of the Sèvres meeting, which remained classified for 35 years, though only out of deference to Eden. "Israel in general and Ben-Gurion in particular had no reason to keep it secret," he said. "It was advantageous to us that the world should know that we were not alone in this aggression."

Mr Logan, the dutiful civil servant, also kept quiet. "I didn't want to be the one blowing the whistle on Eden or Lloyd," he said. But he later obtained a copy of the protocol from the Israelis and deposited it in the National Archives in Kew. It was a monument, in the words of Professor Shlaim, "to French opportunism, Eden's duplicity, and Israeli paranoia".

Every so often, over the years, the Sèvres meeting provoked a flurry of excitement: Dean was exercised about his position when Lloyd published his account of Suez in 1978, according to newly declassified Cabinet Office documents.

Public misled

For Keith Kyle, doyen of Suez historians, the abiding conclusion is this: "It was not ... that the Sèvres meeting was held in secret (that was an accepted device of diplomacy), nor that it was with the Israelis (in 1956 a popular cause in many quarters in Britain and especially with the official opposition), nor even that the policy that it inaugurated was unsuccessful. The case against the proceedings ... has always been twofold: that the British presence was the essential ingredient in the launching of an aggressive war, and that it was the subject of a major misrepresentation to the British public of what British troops were being called upon to do."